Zircon ore is used for the production of zirconium metal. The converting zircon ore (zircon sand) to zirconium metal generally utilizes the following operations: chlorination; separation (to remove hafnium); rechlorination (by processing similar to the initial chlorination); reduction; distillation (MgCl.sub.2 and Mg vaporization for their removal from the zirconium); and double (or triple) arc melting to produce an ingot. The zirconium ingot can be then fabricated into various shapes.
With regard to chlorination, U.S. Pat. No. 4,244,935, issued to Dell on Jan. 13, 1981, relates a method of forming the chloride of a metal-oxygen containing substance based on a fluid coking technique. It should be noted that the commercial process for making zirconium metal utilizes fluidized bed carbochlorination process at about 12000.degree. C. which produces a relatively impure, hafnium-containing zirconium tetrachloride and by-product silicon tetrachloride (which silicon tetrachloride by-product is relatively easily separated by differential condensation). U.S. Pat. No. 3,895,097, issued to Langenhoff et al. on July 15, 1975, also relates to a process for reacting metal oxides with chlorine. Uranium chloride is volatilized at the ore ("crude") chlorinator temperature and thus as an impurity in the zirconium-hafnium tetrachloride stream.
The step is required as naturally occurring zirconium generally contain from 1 to 3 percent hafnium oxide relative to zirconium oxide. In order that the zirconium metal be acceptable as a nuclear reactor material, the hafnium content must first be reduced to low levels, due to the high neutron absorption cross section of hafnium. This separation process is difficult due to the extreme chemical similarity of the two elements. A number of techniques have been explored to accomplish this separation, with the technique currently in use in the United States involving liquid-liquid extraction of aqueous zirconium-hafnium oxychloride thiocyanate complex solution using methyl isobutyl ketone, generally as described in U.S. Pat. No. 2,938,679, issued to Overholser on May 31, 1960, with the removal of iron impurities prior to solvent extraction as described in U.S. Pat. No. 3,006,719, issued to Miller on Oct. 31, 1961. Generally, the uranium impurity partitions between the zirconium stream and the hafnium stream, going principally into the hafnium stream.
Rechlorination (commonly called "pure chlorination"), as noted above, is by processing similar to the initial chlorination. The feed is zirconia (or hafnia), however, rather than the zircon ore, and the chlorination temperature is lower. As a result of the lower temperature, most of the uranium chloride is not volatilized during this chlorination, and generally remains in the residue, and is thus generally separated from the zirconium and hafnium streams.
Commercially, reduction is by reacting gaseous zirconium tetrachloride with molten magnesium to produce zirconium metal (in relatively porous, so-called "sponge", form). Modifications to the reduction process have been suggested in many U.S. Patents, including U.S. Pat. Nos. 4,440,384; 4,511,399; 4,556,420; 4,613,366; 4,637,831; and 4,668,287; assigned to the same assignee.
With regard to "distillation" to remove MgCl.sub.2 and Mg (by their vaporization) from the zirconium sponge, such distillation is usually performed at about 1050.degree.-1100.degree. C. (note, however that Kwon et al in U.S. Pat. No. 4,711,664 teach that iron content can be lowered by distilling at about 934.degree. C.).
Consumable electrode vacuum arc melting is generally used to produce a consolidated ingot from the porous distilled sponge (generally the distilled sponge is broken up and then pressed into disks, for example, which disks are then welded together to form the consumable electrode). An improved consumable electrode is described in Weber's U.S. Pat. No. 4,539,688.
Generally the hafnium is separately, but similarity processed through the reduction through melting stages, except that hafnium is generally melted in an electron beam furnace. In addition, as the uranium from the ore ends up principally in the solid residue of the pure chlorination of hafnium, the hafnium chlorinator residue contains about 5% uranium, which generally precludes recycling of the hafnium chlorination residue.